Need advise for rear speaker switch.

I'm including a video of my dash with what looks like previous owners remedy for a new stereo.
My car is 8 track, 3 speaker dash, rear speaker equipped.
It looks like some wiring was cut out from the drivers side eliminating the rear speakers and switch.
The switch and wiring are still there, but there is a chunk of the green, red, violet and blue missing on the left side towards the right side of the instrument panel by the radio.
Also, I haven't found the crossover either.
I'm including...

Need advise for rear speaker switch.
Author: Curt J